🚀 go-pugleaf

In article <vertigoD...@netcom.com>,
Don Durrance <ver...@netcom.com> wrote:
>Bryan Hitch (hit...@postbox.acs.ohio-state.edu) wrote:
>: >> You will probably find the speed of light to be the same whether in
>: >> vacuum or not... Anyway:
>: >The speed of light varies depending on what medium it travels in.
>: Umm... No, I think you're getting sound and light mixed up.  The speed of 
>: only changes the wavelength (hence the doppler redshift) of the light rays.
>: Try asking any high school physics teacher.
>based the theory of relativity on the fact (?) that light has a constant 
>velocity.

Okay...
Let's try to put this issue to rest:
Light DOES change speed when travelling into different media.
Light tends to travel slower in denser media and faster in undense things
like vaccum or air.
When particles with nonzero restmass are travelling close to the speed of
light in a vaccum (eg electrons) and come to a medium in which the speed of
light is less than their speed, they stop themselves from breaking physical
laws by radiating the excess energy (Cerenkov radiation).

And.. first let us all agree on the fact that at any point along the path of
a beam of light, its frequency must be constant.  This is why light changes
wavelength when travelling into different media on not Doppler shift, which
refers to moving sources of radiation.  Its frequency must remain
constant, but, because its speed changes, its wavelength (which is just
frequency times velocity) must also change.

I'm not entirely sure of why this happens but I think it has something to
do with the fact that the presence of matter curves space a bit.  Either 
that or it's the change in the permittivity and permeability of space in
a medium versus that of free space (vaccum).  The speed of light, after all,
is a function of these two factors (the squareroot of their products or the
reciprocal of that or something).  Suffice it to say, there's a reason
why the make a point of saying "permeability OF FREE SPACE" when defining the
"speed of light IN A VACCUM".

And another thing...  I think you'll find that most high school physics 
teachers have a primary degree in education and not science.  There are 
exceptions, of course, but certainly it means that just 'any high school
physics teacher' wouldn't be much of an authority.

There..I'm done.

-jurg